Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology, such as UMTS (Universal Mobile Communications System), are currently deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE. The work item (WI) specification on LTE called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (Rel-8 LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP TR 25.913, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN),” version 8.0.0, January 2009 (available at http://www.3gpp.org and incorporated herein by reference).
In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in Rel. 8 LTE.
E-UTRAN Architecture
The overall LTE architecture is exemplarily shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of eNBs (sometimes also referred to as eNodeBs), providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the User Equipment (UE). The eNB hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane.
The eNB performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink (UL) Quality-of-Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways (Serving GWs) and eNBs.
The Serving GW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and Packet Data Network Gateway (PDN GW)). For idle state UEs, the Serving GW terminates the downlink (DL) data path and triggers paging when downlink data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the Serving GW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. The MME is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.
OFDM with Frequency-Domain Adaptation
The AML-OFDM-based downlink used in LTE has a frequency structure based on a large number of individual sub-carriers with a spacing of 15 kHz. This frequency granularity facilitates to implement dual-mode UTRA/E-UTRA terminals. The ability to reach high bit rates is highly dependent on short delays in the system and a prerequisite for this is short sub-frame duration. Consequently, the LTE sub-frame duration is set as short as 1 ms in order to minimize the radio-interface latency.
In order to handle different delay spreads and corresponding cell sizes with a modest overhead the OFDM cyclic prefix length can assume two different values. The shorter 4.7 ms cyclic prefix is enough to handle the delay spread for most unicast scenarios. With the longer cyclic prefix of 16.7 ms very large cells, up to and exceeding 120 km cell radius, with large amounts of time dispersion can be handled. In this case the length is extended by reducing the number of OFDM symbols in a sub-frame.
The basic principle of Orthogonal Frequency Division Multiplexing (OFDM) is to split the frequency band into a number of narrowband channels. Therefore, OFDM allows transmitting data on relatively flat parallel channels (subcarriers) even if the channel of the whole frequency band is frequency selective due to a multipath environment. Since the subcarriers experience different channel states, the capacities of the subcarriers vary and permit a transmission on each subcarrier with a distinct data-rate. Hence, subcarrier-wise (frequency domain) Link Adaptation (LA) by means of Adaptive Modulation and Coding (AMC) increases the radio efficiency by transmitting different data-rates over the subcarriers.
OFDM Access (OFDMA) allows multiple users to transmit simultaneously on the different subcarriers per OFDM symbol. Since the probability that all users experience a deep fade in a particular subcarrier is very low, it can be assured that subcarriers are assigned to the users who see good channel gains on the corresponding sub-carriers. When allocating resources in the downlink to different users in a cell, the scheduler takes information on the channel status experienced by the users for the subcarriers into account. The control information signaled by the users, i.e. Channel Quality Index (CQI), allows the scheduler to exploit the multi-user diversity, thereby increasing the spectral efficiency.
Hybrid ARQ Schemes
A common technique for error detection and correction in packet transmission systems over unreliable channels is called Hybrid Automatic Repeat reQuest (HARQ). Hybrid ARQ is a combination of Forward Error Correction (FEC) and the retransmission mechanism Automatic Repeat reQuest (ARQ).
If a FEC encoded packet is transmitted and the receiver fails to decode the packet correctly (errors are usually checked by a CRC (Cyclic Redundancy Check)), the receiver requests a retransmission of the packet. Generally, the transmission of additional information is called “retransmission (of a data packet)”, although this retransmission does not necessarily mean a transmission of the same encoded information, but could also mean the transmission of any information belonging to the packet (e.g. additional redundancy information).
Depending on the information (generally code-bits/symbols), of which the transmission is composed of, and depending on how the receiver processes the information, the following hybrid ARQ schemes are defined:
HARQ Type I
If the receiver fails to decode a data packet correctly, the information of the encoded data packet is discarded and a retransmission is requested. This implies that all transmissions of the data packet are decoded separately. Generally, retransmissions contain identical information (code-bits/symbols) to the initial transmission of the data packet.
HARQ Type II
If the receiver fails to decode a data packet correctly, a retransmission of the data packet is requested, where the receiver stores the information of the (erroneous received) encoded data packet as soft information (soft-bits/symbols). This implies that a soft-buffer is required at the receiver. Retransmissions can be composed out of identical, partly identical or non-identical information (code-bits/symbols) according to the same data packet as earlier transmissions.
When receiving a retransmission the receiver combines the stored information from the soft-buffer and the currently received information and tries to decode the data packet based on the combined information. The receiver can also try to decode the transmission individually, however generally performance increases when combining transmissions. The combining of transmissions is also referred to as soft-combining, where multiple received code-bits/symbols are likelihood combined and solely received code-bits/symbols are code combined. Common methods for soft-combining are Maximum Ratio Combining (MRC) of received modulation symbols and log-likelihood-ratio (LLR) combining (LLR combing only works for code-bits).
Type II schemes are more sophisticated than Type I schemes, since the probability for correct reception of a packet increases with receive retransmissions. This increase comes at the cost of a required hybrid ARQ soft-buffer at the receiver. This scheme can be used to perform dynamic link adaptation by controlling the amount of information to be retransmitted. E.g. if the receiver detects that decoding has been “almost” successful, it can request only a small piece of information for the next retransmission (smaller number of code-bits/symbols than in previous transmission) to be transmitted. In this case it might happen that it is even theoretically not possible to decode the packet correctly by only considering this retransmission by itself (non-self-decodable retransmissions).
HARQ Type III
This is a subset of Type II with the restriction that each transmission must be self-decodable.
HARQ Protocol Operation for Unicast Data Transmissions in LTE
In LTE there are two levels of re-transmissions for providing reliability, namely, HARQ at the MAC layer and outer ARQ at the RLC layer. The outer ARQ is required to handle residual errors that are not corrected by HARQ that is kept simple by the use of a single bit error-feedback mechanism, i.e. ACK/NACK.
An N-process stop-and-wait HARQ is employed that has asynchronous re-transmissions in the downlink and synchronous re-transmissions in the uplink. Synchronous HARQ means that the re-transmissions of HARQ blocks occur at predefined periodic intervals. Hence, no explicit signaling is required to indicate to the receiver the retransmission schedule. Asynchronous HARQ offers the flexibility of scheduling re-transmissions based on air interface conditions. In this case some identification of the HARQ process needs to be signaled in order to allow for a correct combing and protocol operation.
In 3GPP, HARQ operation with eight processes is decided for LTE. The HARQ protocol operation for downlink data transmission is similar or even identical to HSDPA. In uplink HARQ protocol operation there are two different options on how to schedule a retransmission. Retransmissions are either scheduled by a NACK (=synchronous non-adaptive retransmission) or explicitly scheduled by a Physical Downlink Control CHannel (PDCCH) (=synchronous adaptive retransmissions).
In case of a synchronous non-adaptive retransmission the retransmission will use the same parameters as the previous uplink transmission, i.e. the retransmission will be signaled on the same physical channel resources respectively uses the same modulation scheme. Since synchronous adaptive retransmission are explicitly scheduled via PDCCH, the eNB has the possibility to change certain parameters for the retransmission. A retransmission could be for example scheduled on a different frequency resource in order to avoid fragmentation in the uplink, or the eNB could change the modulation scheme or alternatively indicate to the UE what redundancy version to use for the retransmission. It should be noted that the HARQ feedback (ACK/NACK) and PDCCH signaling occurs at the same timing. Therefore the UE only needs to check once whether a synchronous non-adaptive retransmission is triggered, only NACK is received, or whether eNB requests a synchronous adaptive retransmission, i.e. a PDCCH is signaled in addition to the HARQ feedback.
L1/2 Control Signaling in LTE
In order to inform the scheduled users about their allocation status, transport format and other data related information (e.g. HARQ), L1/L2 control signaling is transmitted on the downlink along with the data. The control signaling is commonly multiplexed with the downlink (user) data in a sub-frame (assuming that the user allocation can change from sub-frame to sub-frame). Generally, it should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, then the L1/2 control signaling needs only be transmitted once per TTI.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). It should be noted that assignments for uplink data transmissions, uplink grants, are also transmitted on the PDCCH.
Generally, the information sent on the L1/L2 control signaling may be separated into the Shared Control Information (SCI) and Dedicated Control Information (DCI).
Shared Control Information (SCI)
Shared Control Information (SCI) carries so-called Cat 1 information. The SCI part of the L1/L2 control signaling contains information related to the resource allocation (indication). The SCI typically contains the following information:                User identity, indicating the user which is allocated        RB allocation information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Note, that the number of RBs on which a user is allocated can be dynamic.        Duration of assignment (optional) if an assignment over multiple sub-frames (or TTIs) is possible        
Depending on the setup of other channels and the setup of the Dedicated Control Information (DCI), the SCI may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Dedicated Control Information (DCI)
Dedicated Control Information (DCI) carries the so-called Cat 2/3 information. The DCI part of the L1/L2 control signaling contains information related to the transmission format (Cat 2) of the data transmitted to a scheduled user indicated by Cat 1. Moreover, in case of application of (hybrid) ARQ it carries HARQ (Cat 3) information. The DCI needs only to be decoded by the user scheduled according to Cat 1. The DCI typically contains information on:                Cat 2: Modulation scheme, transport-block (payload) size (or coding rate), MIMO related information, etc. Note, either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated RBs).        Cat 3: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence numberL1/L2 Control Signaling Information for Downlink Data Transmission        
Along with the downlink packet data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH). This L1/L2 control signaling typically contains information on:                The physical channel resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the UE (receiver) to identify the resources on which the data is transmitted.        The transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the UE (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. In some cases the modulation scheme maybe signaled explicitly.        HARQ information:                    Process number: Allows the UE to identify the HARQ process on which the data is mapped.            Sequence number or new data indicator: Allows the UE to identify if the transmission is a new packet or a retransmitted packet.            Redundancy and/or constellation version: Tells the UE, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation)                        UE Identity (UE ID): Tells for which UE the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other UEs to read this information.L1/L2 Control Signaling Information for Uplink Data Transmission        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the UE about the transmission details. This L1/L2 control signaling typically contains information on:                The physical channel resource(s) on which the UE should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        The transport Format, the UE should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the UE (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    Process number: Tells the UE from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the UE to transmit a new packet or to retransmit a packet.            Redundancy and/or constellation version: Tells the UE, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        UE Identity (UE ID): Tells which UE should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other UEs to read this information.        
There are several different flavors how to exactly transmit the information pieces mentioned above. Moreover, the L1/L2 control information may also contain additional information or may omit some of the information. E.g.:                HARQ process number may not be needed in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as e.g. pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (for transmissions on the Physical Uplink Shared CHannel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The TF respectively modulation and coding scheme (MCS) field has for example a size of bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating RVs 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. For details on the TBS/RV signaling for uplink assignments on PDCCH please see 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 8.5.0, December 2008 (available at http://www.3gpp.org and incorporated herein by reference). The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07) last November [Final Acts WRC-07, Geneva, November 2007]. Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved 3GPP TR 36.814 (available at http://www.3gpp.org and incorporated herein by reference). The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
LTE-A Support of Wider Bandwidth
Carrier aggregation, where two or more component carriers are aggregated, is considered for LTE-Advanced in order to support wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation.
A terminal may simultaneously receive or transmit one or multiple component carriers depending on its capabilities:                An LTE-Advanced terminal with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers.        An Rel-8 LTE terminal can receive and transmit on a single component carrier only, provided that the structure of the component carrier follows the Rel-8 specifications.        
It shall be possible to configure all component carriers LTE Release 8 compatible, at least when the aggregated numbers of component carriers in the UL and the DL are same. Consideration of non-backward-compatible configurations of LTE-A component carriers is not precluded
LTE-A Support of Relaying Functionality
Relaying is considered for LTE-Advanced as a tool to improve e.g. the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas.
The relay node is wirelessly connected to radio-access network via a donor cell. The connection can be                inband, in which case the network-to-relay link share the same band with direct network-to-UE links within the donor cell. Rel-8 UEs should be able to connect to the donor cell in this case.        outband, in which case the network-to-relay link does not operate in the same band as direct network-to-UE links within the donor cell        
With respect to the knowledge in the UE, relays can be classified into transparent, in which case the UE is not aware of whether or not it communicates with the network via the relay, and non-transparent, in which case the UE is aware of whether or not it is communicating with the network via the relay.
Depending on the relaying strategy, a relay may be part of the donor cell or control cells of its own.
In the case the relay is part of the donor cell, the relay does not have a cell identity of its own (but may still have a relay ID). At least part of the radio resource management (RRM) is controlled by the eNB to which the donor cell belongs, while parts of the RRM may be located in the relay. In this case, a relay should preferably support also Rel-8 LTE UEs. Smart repeaters, decode-and-forward relays and different types of Layer 2 relays are examples of this type of relaying.
In the case the relay is in control of cells of its own, the relay controls one or several cells and a unique physical-layer cell identity is provided in each of the cells controlled by the relay. The same RRM mechanisms are available and from a UE perspective there is no difference in accessing cells controlled by a relay and cells controlled by a “normal” eNodeB. The cells controlled by the relay should support also Rel-8 LTE UEs. Self-backhauling (Layer 3 relay) uses this type of relaying.
In FIG. 3 shows an exemplary LTE-A system which utilizes relay nodes (RN). The wireless interface between eNB and RN, which connects a RN with the radio access network, is referred to as Y3 interface.
It would be in general advantageous to utilize in LTE-A the presence of relay nodes in order to benefit from macro-diversity downlink transmissions. Since it is presently foreseen that relay nodes support scheduling and HARQ functionalities (data of) the same transport block could be transmitted simultaneously from multiple network nodes, i.e. eNB and RN. By combining the received energy from multiple received downlink transmissions prior to decoding, the receiver in the UE could benefit from the macro-diversity gain and hence increased decoding performance. This would be similar to the soft handover operation in UMTS. UMTS supports soft handover operation for downlink as well as for uplink in order to obtain the macro-diversity gain.
However only for the uplink direction, i.e. in Rel-6 HSUPA, HARQ is also supported for soft handover. For the downlink direction, there is no support of HARQ in soft handover. HSDPA (Rel-5/Rel-7) does not support soft handover due to problems with the HARQ protocol. Since for a downlink HARQ operation during soft handover, there will be multiple HARQ transmitting entities and single HARQ receiving entity, HARQ feedback errors on the uplink direction could lead to unsynchronized HARQ protocol states.
For example, in case UE sends an NACK in response to the reception of downlink transmissions from the two eNBs forming the UE's active set, and assuming that one of the eNBs erroneously detects an ACK due to errors on the uplink channel, the HARQ protocol states in the eNBs would be different, i.e. the eNB receiving the NACK assumes a further retransmission for the transport block, whereas the eNB erroneously detecting an ACK assumes that the transport block was correctly decoded by UE and starts transmitting a new transport block.